1. Field of the Invention
This invention relates to illumination for image acquisition devices, and more particularly, to illumination for image acquisition devices used for reading industrial identification codes.
2. Description of the Related Art
Machine vision systems use image acquisition devices that include camera sensors to deliver information on a viewed subject. The system then interprets this information according to a variety of algorithms to perform a programmed decision-making and/or identification function. For an image to be most effectively acquired by a sensor in the visible, and near visible light range, the subject should be properly illuminated.
In the example of identification symbol scanning using an image sensor, good lighting is highly desirable. Identification symbol scanning entails the aiming of an image acquisition sensor (CMOS camera, CCD, etc.) at a location on an object that contains a bar code, data matrix, or other identification symbol, and retrieval of an image of that symbol. The identification symbol contains a set of predetermined patterns that represent an ordered group of characters or symbols from which an attached data processor (for example a microcomputer) can derive useful information about the object (e.g. its serial number, type, model, price, etc.). Identification symbols are available in a variety of shapes and sizes. Two of the most commonly employed symbol types are the so-called one-dimensional barcode, consisting a line of vertical stripes of varying width and spacing, and the so-called two-dimensional barcode consisting of a two-dimensional array of dots or rectangles.
In reading identification symbols or other subjects of interest the type of illumination employed is of particular concern. Where barcodes and other viewed subjects are printed on a flat surface with contrasting ink or paint, a diffuse, high-angle “bright field” illumination may best highlight these features for the sensor. By high-angle it is meant, generally, light that strikes the subject nearly perpendicularly (normal) or at an angle that is typically no less than about 45 degrees from perpendicular (normal) to the surface of the item being scanned. Such illumination is subject to substantial reflection back toward the sensor. By way of example, barcodes and other subjects requiring mainly bright field illumination may be present on a printed label adhered to an item or container, or on a printed field in a relatively smooth area of item or container.
Conversely, where a barcode or other subject is formed on a more-irregular surface or is created by etching or peening a pattern directly on the surface, the use of highly reflective bright field illumination may be inappropriate. A peened/etched surface has two-dimensional properties that tend to scatter bright field illumination, thereby obscuring the acquired image. Where a viewed subject has such decidedly two-dimensional surface texture, it may be best illuminated with dark field illumination. This is an illumination with a characteristic low angle (approximately 45 degrees or less, for example) with respect to the surface of the subject (i.e. an angle of more than approximately 45 degrees with respect to normal). Using such low-angle, dark field illumination, two-dimensional surface texture is contrasted more effectively (with indents appearing as bright spots and the surroundings as shadow) for better image acquisition.
To take full advantage of the versatility of a camera image sensor, it is desirable to provide both bright field and dark field illumination for selective or simultaneous illumination of a subject. However, dark field illumination must be presented close to a subject to attain the low incidence angle thereto. Conversely, bright field illumination is better produced at a relative distance to ensure full area illumination.
In the situation where a barcode or other subject is formed on a specular surface, non-directional totally diffuse bright field illumination is preferred. This type of illumination, like that naturally produced on a cloudy day must surround the object upon which the barcode is placed. In typical machine vision applications, non-directional totally diffuse illumination is difficult and costly to provide, usually by way of surrounding a workpiece with a light tent.